1. Field
The invention is in the field of methods and apparatus for recovery of oil from underground oil-bearing formations by the introduction thereinto, and continuous flowing therethrough, of a heated non-aqueous gas.
2. State of the Art
An appendix of terms is included at the end of this specification.
Traditionally, oil has been produced from underground oil-bearing formations by a series of processes, the first of which is called the primary phase of oil production. This phase utilizes natural stored energy in or near the formation to move the oil to production wells, and to bring it to the surface, assisted as needed by pumping. When the natural energy is depleted a secondary phase is entered into whereby water under pressure is injected into the formation to supply supplemental energy and to thus force additional oil to move to the wells. When production by this method ceases to be economical a tertiary phase is entered into, spoken of as enhanced oil recovery (EOR). In this phase a variety of methods are employed, such as chemical flooding, gas flooding, steam flooding, in situ combustion, thermal methods, and various combinations of these. A summary of such methods is given in a publication by Herbeck, et al., entitled "Fundamentals of Tertiary Oil Recovery", Energy Communications, Dallas, Tex., 1977.
The combined total oil production by the primary and secondary phases is generally less than 40% of the original oil-in-place. The tertiary phase may recover an additional 10%. Thus, existing methods of oil recovery leave approximately 50% of the oil-in-place behind in the formation. Some experts have predicted that unless some new techniques are developed it will be necessary, about the year 2000 A.D., to mine the formations to recover this oil. This will be very costly since the formations are generally 1,000 to 10,000 feet below the surface of the ground.
The above processes are normally employed in formations in which the viscosity of the oil is relatively low. Recovery of highly viscous oil by these procedures is usually not economical and such procedure cannot be used for the recovery of oil locked in so-called tar sands.
Flooding methods referred to above involve subjecting the matrix to a flood of fluid, under pressure, so as to force the oil from an injection well to a production well. Substantially the entire depth of the matrix in a vertical cross section is subjected to this pressurized flood.
Chemical or micellar solution flooding involves preinjection of a fluid to condition the deposit, followed by injection of a flooding alkaline or alkaline polymer solution that forms surfactants in situ for releasing oil, followed by a polymer solution for mobility control, and a driving fluid, usually water, to move the chemicals and resulting oil to the production well. Pollution problems are severe with this process due to the storage and use of large quantities of polluting chemicals. In addition, the cost is high.
The usual carbon dioxide flooding involves injecting a slug of CO.sub.2 followed by alternate water and CO.sub.2 injections. The objective is to attain a miscible zone of CO.sub.2 and oil at their interface. Otherwise, interfacial tensions cause the oil to form in microscopic droplets which tend to bind to the underground material in the pores thereof. However, CO.sub.2 and oil are not miscible at first contact. Dynamic miscibility occurs when the density of the CO.sub.2 is sufficiently great that hydrocarbons of the oil are solubilized in the CO.sub.2. This requires pressures of 1500 psia to 6000 psia. Also, the oil must have a gravity above about 25.degree. API. A problem also arises from the precipitation of asphaltenes and paraffins due to compositional disturbances caused by the miscible displacement. Such precipitation creates plugging and clogging problems.
Steam flooding involves injecting steam, which condenses to hot water at the interface with the deposit. Being reinforced by steam pressure behind it, the hot water tends to push the oil through the deposit to the production well. This is a popular process but leaves much oil unrecovered.
Cyclic steam stimulation is basically a thermal process wherein steam is injected into a deposit of highly viscous oil in order to introduce heat. The formation is allowed to soak for several days, during which time some of the oil is thinned. The thinned oil is then pumped out of the same well.
Another thermal process involves boring holes in the formation and placing electrical heaters therein to heat the oil for reducing its viscosity. This is a clostly procedure.
Still another thermal process proposed for extracting oil from oil shales involves injecting CO.sub.2 that is heated to a temperature of at least 1000.degree. F. and pressurized to a pressure of at least 500 psig into an injection well so as to heat the shale and thus to vaporize and/or entrain the hydrocarbons that result from cracking and/or vaporization and liquefaction and to produce from the formation through a recovery well. This process has been known for many years, but apparently has not been successful.
Yet another thermal process, known as in-situ combustion, involves igniting the oil in a fire flood at an injection well, and, by continued injection of air, causing a fire front to advance through the deposit to a production well.
However, all of these methods and variations thereof leave a large quantity of oil behind as unrecovered. Moreover, many of them require the use of high pressures in the thousands of psia range, which cannot be used in shallow formations due to the danger of fracturing the impervious overburden.
Again, a problem lies in the requirement to maintain a favorable mobility ratio. The ratio of the mobility of the flooding substance to the mobility of the oil must be less than 10, preferably near 1.
Another difficulty lies in the need for very high temperatures, in excess of 1000.degree. Fahrenheit, which is very costly. In addition, for some deposits this results in calcining which causes a prodigious waste of heat.
Mention should also be made of another process involving mining the oilbearing material. The mined material is retorted so as to recover the oil by distillation. This is not economical for most formations and for the obtaining of low priced oil products.
3. Objectives
Principal objectives in the making of the present invention were to develop method and apparatus for substantially increasing the amount of oil-in-place recovered over existing secondary and tertiary methods for oil fields wherein the oil has been depleted to the point that production is no longer economical, and also for extracting highly viscous oil from tar sands and other formations that are not economically processed by existing methods.